Photovoltage spectroscopy

Photovoltage spectroscopy is used less frequently than photocurrent spectroscopy, but it has played an important role in commercial semiconductor characterisation systems. It has been used, or example, to determine the bandgap of ternary or quaternary III-V semiconductors. The main drawback of the method is that the relationship between the photovoltage and the absorbed photon flux is non-linear. However, it can be shown that near the band edge (small absorption coefficient), the photovoltage is a linear function of the absorption coefficient a. Figures 12.11 and 12.12 illustrate the use of photovoltage spectroscopy to determine the bandgap (and hence the compos­ition) of epitaxial layers of Ga1-xAlxAs alloy samples (Hutton and Peter, 1993).

Photovoltage spectra are measured at open circu...

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Photocurrent, photovoltage and microwave reflectance methods

12.3.1 Photocurrent spectroscopy

When a semiconductor/electrolyte junction is illuminated, electrons can be excited from the valence band to the conduction band provided that the photon energy hv exceeds the bandgap energy Eg. In addition, photons with lower energies may excite electrons or holes from surface or bulk traps to the bands, giving a sub-bandgap photoresponse. Under depletion conditions, minority carriers (holes in n-type and electrons in p-type semiconductors) created by light absorption in the space-charge region are transported rapidly (<10-9 s) to the interface by drift/diffusion...

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Electrochemical impedance spectroscopy (EIS)

Many of the problems of interpretation that arise from single-frequency admittance measurements can be avoided if frequency-resolved measurements are made (Gomes and Vanmaekelbergh, 1996; Macdonald et al., 1998). These can be carried out with a programmable lock-in amplifier or using a frequency response analyser (FRA) with its associated electrochemical interface. FRA systems are optimised to overcome the problems associated with attenuation and phase shift. The current to the cell is measured using a series resistance and a wide-bandwidth voltage amplifier, and the AC voltage appearing across the cell between the working and reference electrodes is measured directly to avoid errors arising from the performance of the potentiostat control amplifier...

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Electrical methods

12.2.1 Single – frequency admittance measurements (Mott-Schottky plots)

Solid-state semiconductor junctions are commonly characterised by AC and DC electrical techniques. In the case of Schottky barrier (metal/semiconductor) devices, single admittance measurements (‘C-V measurements’) are often used to determine the space-charge layer capacitance Csd as a function of applied voltage bias V. Analysis of the potential dependence of Cscl provides values of the barrier height and doping density. This method has been applied widely in semiconductor electrochemistry to determine flatband potentials and doping densities (Morrison, 1980). The semiconductor may be in contact with either a redox electrolyte or an inert supporting electrolyte...

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Department of Chemistry, University of Bath, Bath BA2 7AY, UK
l. m.peter@bath. ac. uk


Department Solare Energetik, Hahn Meitner Institut, D-1409 Berlin, Germany

Tributsch @ hmi. de

Give me the splendid silent sun with all his beams full-dazzling. Walt Whitman, Give Me the Splendid Silent Sun, 1900

12.1 Introduction

Photoelectrochemistry is an interdisciplinary subject that brings together different aspects of solid-state physics, electronics and chemistry. This interdisciplinarity, which is part of the fascination of photoelectrochemistry, is reflected in the unusually wide range of techniques employed to characterise photoelectrochemical systems...

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Electron injection in many dye-sensitised metal oxides can occur on the ultrafast time scale. This process has been extensively studied in recent years because of its important role in determining solar cell performance. Among the many available ultrafast techniques, transient absorption in the visible and mid-IR are the most commonly used because of their ability to monitor both the reactants and products of ET on the ultrafast time scale. From extensive studies by many groups, a qualitative understanding of interfacial electron-injection dynamics from adsorbate molecules to semiconductors has emerged...

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Dependence on the semiconductor

In addition to TiO2, there is also considerable interest in dye-sensitised solar cells based on other metal oxides, such as ZnO (Rensmo et al., 1997; Sayama et al., 1998; Hara et al., 2000), Nb2O5 (Hu et al., 1995; Barros Filho et al., 1997; Sayama et al., 1998; Hara et al., 2000; Aegerter, 2001; Lenzmann et al., 2001; Aegerter et al., 2002), SnO2 (Bedja et al., 1994; Ferrere et al., 1997; Hara et al., 2000), WO3 (Sayama et al., 1998), Ta2O5 (Sayama et al., 1998) and In2O3 (Sayama et al., 1998; Hara et al., 2000). However, so far the reported conversion efficiencies are significantly lower than those based on N3-sensitised TiO2 (Nazeeruddin et al., 1993). The reasons for these lower conversion efficiencies remain unclear...

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Dependence on electronic coupling

Подпись: k ET (Г ) image804 Подпись: (11.9)

The strength of the electronic coupling between the molecular adsorbate and the semiconductor depends on the electronic structure of the donor molecule, the spacers and the anchoring group, and the semiconductor. One of the most convenient ways to vary the coupling strength systematically is to control the bridge length between the chromophore and the binding group. This approach was used successfully in earlier studies of thermal interfacial ET on metal or semiconductor electrodes modified by self-assembled-monolayers (SAM) of alkanethiols (Chidsey, 1991; Becka and Miller, 1992; Finklea and Hanshew, 1992; Smalley et al., 1995; Sachs et al., 1997; Gu and Waldeck, 1998; Creager et al., 1999; Smalley et al., 2003)...

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